The present invention relates to pressure sensitive adhesives and more particularly to a water-based pressure sensitive adhesive suitable for high performance applications.
The manufacture of pressure sensitive adhesive tapes is being shifted from organic solvent systems to aqueous latices in order to reduce air pollution and fire hazard, as well as eliminate the rising expense of organic solvents. Pressure sensitive adhesives for high performance application must meet or exceed diverse, often seemingly incompatible, criteria. Initially, the adhesive must coat well on silicone or other release sheets. The adhesive also must exhibit tenacious adherence to film face stock which may serve a masking function or may convey information, e.g. by its imprinting.
When the adhesive coated film face stock is removed from the release sheet, the adhesive must exhibit initial tack and peel performance when pressure-applied to a substrate. The substrate beating the adhesively-bound film face stock may be subjected to a variety of manufacturing operations over time. Thermal cycling of the substrate is a condition to which high performance, pressure sensitive adhesives must be designed to confront. When the film face stock is removed from the substrate, removal must be clean, i.e. no visible trace of the adhesive should remain on the substrate. Heat aging of the film face stock/substrate laminate, however, tends to cause the adhesive to "build", that is, the necessity to utilize excessive force to remove the film face stock. Thus, the adhesive should stick well initially, but not so well that its later removal is difficult. Then too, the adhesive must possess shelf life, viscosity, etc. making it compatible with existing coating and handling techniques utilized in the art.
Prior proposals aimed at making pressure sensitive adhesives tapes include, for example, U.S. Pat. No. 3,657,396 which proposes a mixture of polyvinylmethyl ether and an acrylic acid polymer which mixture is dispersed in water. The acrylic polymer is selected from the group consisting of polyacrylic acid, polyacrylamide, N-methylol polyacrylamide and an acrylic copolymer of a monomer selected from the group consisting of acrylic acid, acrylamide, and N-methylol acrylamide in a monomer selected from the group consisting of lower alkyl esters of acrylic acid and methacrylic acid. European patent application publication No. 287306 proposes removable pressure-sensitive adhesive tapes comprising copolymers of terminally-unsaturated vinyl monomers and vinyl-unsaturated homopolymerizable emulsifier monomers, combined with a phosphate emulsifier. Japanese Kokai patent No. Sho 47 [1972]-110263 proposes a transfer-resistant aqueous pressure sensitive adhesive comprising a resin emulsion derived from an acrylic monomer and 1-4 weight pans of an ethylenically-unsaturated acrylamide monomer. Australian application No. 24491/84 proposes pressure sensitive adhesives comprising an acrylic copolymer synthesized from 2-ethylhexyl acrylate and n-butyl acrylate, acrylonitrile, acrylic acid, N-methylol acrylamide, and optionally vinyl sulfonic acid; and plasticizer.
U.S. Pat. No. 4,540,739 proposes a pressure sensitive adhesive (PSA) having improved shear properties and comprising a latex of a polymer having a T.sub.g which is less than about 0.degree. C. and formed by emulsion polymerizing a monomeric mixture of 60-99.5 wt-% of a C.sub.1-C.sub.8 alkyl or alkanol ester of (meth)acrylic acid and from about 0.5 to about 40 wt-% of a C.sub.3 -C.sub.9 ethylenically unsaturated acid. Optionally, the monomeric mixture can include up to 5 wt-% of an amide of a C.sub.3 -C.sub.9 ethylenically unsaturated acid which amide may be unsubstituted or substituted at the nitrogen atom by a C.sub.1 -C.sub.8 alkyl or alkanol radical. The PSA has its pH adjusted to be equal to or greater than about 6 with an agent selected from hydroxides and salts of lithium, sodium, potassium, and mixtures thereof.